The invention relates to a plasma jet spark plug for internal combustion engines having a center electrode, a shot channel defined by insulation material, and a ground electrode concentric with the shot channel and forming an outlet opening of the shot channel.
With efforts to reduce fuel consumption and pollution emissions by internal combustion engines, there are demands for using a leaner fuel-air mixture (fuel-air mixtures with an air-to-fuel ratio greater than one). This requires the creation of highly effective spark plasmas, which can effectively initiate the combustion of such leaner mixtures.
A spark plug having a central electrode, a shot channel, and a ground electrode has been disclosed in an internet publication by RWTH Aachen, which is accessible on the Internet at http://www.vka.rwth-aachen.de/sfb 224/Kapitel/pdf/kap3 2.pdf.
The RWTH Aachen spark plug is capable of generating a plasma outside the spark plug. However, most of the spark energy is not transmitted to the gas. The depth of penetration of the spark plasma into the gas is low. Therefore, the RWTH Aachen spark plug has only a limited ability to ignite lean fuel-air mixtures.
The object of the present invention is to create a spark plug capable of transmitting most of the spark energy to the fuel-air mixture.
According to the invention, the center electrode of the spark plug is conically shaped and the shot channel has a taper which acts as an acceleration zone for the plasma in its path toward the ground electrode.
The conical shape of the center electrode facilitates the development of a plasma. The design of the shot channel and the acceleration zone effective in its path toward the ground electrode ensure deep penetration of the plasma into the fuel-air mixture and, consequently, an optimum ignition effect even with an extremely lean fuel mixture.